Scientists studying the massive earthquake that struck the South Pacific on September 29, 2009, have found that it actually involved two great earthquakes: an initial one with magnitude 8.1, which then triggered another magnitude 8 earthquake seconds later on a different fault. The details of this rare event, called a "triggered doublet," are unlike anything seismologists have seen before.

"We know of no precedent for the Samoa triggered doublet," said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz, who led a seismological analysis of the event published in the August 19 issue of Nature.

The earthquakes unleashed devastating tsunami waves that swept onto the islands of Samoa, American Samoa, and Tonga, killing 192 people. It took months, however, for seismologists to make sense of the confusing seismic data and figure out exactly what happened in the Earth's crust to cause this disaster.

Most great earthquakes (earthquakes of magnitude 8 or greater) occur in subduction zones, where one plate of the Earth's crust dives beneath another plate. The Tonga subduction zone in the South Pacific marks the boundary where the Pacific plate is sinking under the Australian plate.

In the sequence of events on September 29, the first earthquake actually occurred not at the subduction zone, but within the Pacific plate at a site 50 to 100 kilometers (30 to 60 miles) east of the plate boundary. The rupture occurred along an extensional or "pull-apart" fault in the middle of the plate. Such large extensional faulting near a subduction zone is rare, and this is the third largest such event recorded in the 110-year history of seismological monitoring.

According to Lay, these events seem to occur in areas where the plate boundary is relatively weak and one plate slides easily under the other. As the leading edge of the oceanic plate sinks into the mantle, it pulls on the rest of the plate, bending it downward and causing it to break along the extensional fault.

"We think the subducted slab is pulling on the Pacific plate, and that 'slab pull' is responsible for the bending and extension of the plate manifested in the Samoa earthquake," he said.

As seismologists in several programs, including Lay's team, began to study the event in more detail, they noticed some strange inconsistencies. Aftershocks were spread over a huge area, including the Tonga subduction zone, with relatively few along the fault in the Pacific plate that had ruptured. In addition, when researchers used different methods to calculate the fault geometry, they came up with inconsistent solutions, which is rare for large earthquakes. Finally, tsunami prediction models gave results that were not entirely consistent with observations from ocean buoys and on the islands.

At a meeting of the American Geophysical Union in December, Chen Ji of UC Santa Barbara, who was not a member of Lay's team, suggested that the event involved two earthquakes on different faults with different geometries. Lay said he immediately began exploring the idea proposed by Ji.

"The huge signals from the extensional faulting made it difficult to resolve the triggered event, but we were able to confirm that secondary faulting had occurred, that it involved thrust faulting in the Tonga subduction zone, and that it released energy from about 50 to 130 seconds after the onset of extensional faulting within the Pacific plate," Lay said.

The strong shaking from the initial fault rupture appears to have triggered the second event, which involved two major subevents of magnitude 7.8, with a total magnitude equal to 8.0. This second event was followed by many aftershocks in the subduction zone. It also produced a tsunami wave that interfered with and complicated the wave pattern generated by the initial event. And it made the seismic wave analysis very difficult.

"It's impressive that we could have a magnitude 8 earthquake and not recognize it, but this one occurred so quickly after the first that it was not observed by the normal procedures," Lay said.

According to Lay, there is one prior case, in the Kuril Islands, of a pair of great earthquakes that included a subduction zone rupture and extensional faulting in the oceanic plate. In that case, however, the initial event occurred on a thrust fault in the Kuril subduction zone on November 15, 2006 (magnitude 8.4). Two months later, on January 13, 2007, the Pacific plate ruptured in a magnitude 8.0 extensional event. In that case, the slippage on the thrust fault put added stress on the extensional fault, which eventually ruptured.

"That type of behavior has been observed many times, but usually the late extensional faulting is much smaller than the thrusting event," Lay said.

In the Samoa-Tonga event, the extensional faulting occurred first and triggered the thrust fault, with a delay of less than a minute. The location of the thrusting event was not right next to the extensional faulting, but farther south along the Tonga subduction zone.

"In order for slab-pull to have loaded the extensional stress within the Pacific plate, the region of the subduction zone right next to the first event had to have low friction," Lay said. "The sudden extensional motion then produced strong vibrations that shook the plate boundary to the south and caused the abrupt thrusting motion."

Without the transient high strains from the shaking of the first earthquake, that region might have slipped slowly rather than abruptly. According to Lay, there is no record of prior great thrusting earthquakes in this region. "This is a state called conditional stability; the region would normally have episodic slow sliding without earthquakes, but if accelerated it becomes a normal fast-sliding zone," he said.

Earthquake triggering greatly complicates earthquake forecasting and rapid-warning procedures, Lay said. "There may be far more interaction between earthquakes than we have understood," he said. "Studying these complexities is essential for improving our understanding of how earthquakes rupture, how they interact, and how we can mitigate their impact on humans."

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In addition to Lay, the coauthors of the paper include Charles Ammon of Pennsylvania State University; Hiroo Kanamori of the California Institute of Technology; Luis Rivera of the University of Strasbourg, France; Keith Koper of St. Louis University; and Alexander Hutko of the U.S. Geological Survey. This research was supported by the National Science Foundation and the U.S. Geological Survey.

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